Work vehicles often include a parking brake having a spring applied, hydraulic release design. Thus, when the supply of hydraulic fluid is cut-off from the brake's actuator or fluid is being supplied to the actuator at a relatively low pressure, a biasing spring within the actuator forces the parking brake into an engaged state. In contrast, when hydraulic fluid is being supplied to the actuator at a relatively high pressure, the force provided by such high pressure fluid is sufficient to overcome the biasing force provided by the spring, thereby allowing the parking brake to be disengaged.
Typically, the supply of hydraulic fluid to the parking brake is controlled via a simple on/off valve, such as a single-coil solenoid activated on/off valve. Thus, by energizing and dc-energizing the single coil, the valve may be moved between its open and closed positions. In the open position, hydraulic fluid may be supplied to the parking brake at a sufficient pressure to overcome the biasing force provided by the spring and, thus, disengage the brake. Similarly, in the closed position, the supply of hydraulic fluid to the parking brake is cut-off, thereby allowing the brake to be engaged.
While the above-described braking system is effective in allowing the parking brake to be engaged/disengaged under most circumstances, the system is not well-equipped to handle electrical faults and/or other system faults. For example, with a spring applied, hydraulic release brake, the on/off valve may be no longer capable of maintaining the brake disengaged when a wiring or coil fault associated with the valve occurs. As a result, the parking brake may be inadvertently engaged while the vehicle is being driven. Other conventional braking systems suffer the same type of drawback, with each failing to provide an effective means for maintaining a parking brake in its current state despite an electrical fault or other system fault.
Accordingly, an improved braking system that is designed to maintain functionality despite the occurrence of electrical faults or other system faults would be welcomed in the technology.